1. Field of the Invention
The present invention relates to a steering system in a wheeled vehicle, and more particularly to a four-wheel steering system in a wheeled vehicle the front-wheel steering mechanism of which is drivingly interconnected to a rear-wheel steering mechanism by means of a mechanical linkage to steer the dirigible rear road wheels in dependence upon the turn-angle of the steering wheel of the vehicle.
2. Description of the Prior Art
In Japanese Patent Early Publication No. 60-64073, there has been proposed a four-wheel steering system in a wheeled vehicle the front-wheel steering mechanism of which is drivingly interconnected to a rear-wheel steering mechanism by means of a mechanical linkage composed of front and rear connecting shafts. Disposed between the front and rear connecting shafts is a coupling which includes drive and driven members assembled in such a manner as to establish torque transmission between the connecting shafts only when the front connecting shaft is rotated at a large angle. In the coupling, the drive member is circumferentially spaced in a predetermined distance from the driven member to make the torque transmission between the connecting shafts ineffective when the front connecting shaft is rotated at a small angle. Under control of the coupling, the rear road wheels are maintained in their neutral positions when the front road wheels are steered at a small angle and steered in the opposite direction relative to the front road wheels when the front road wheels are steered at a large angle. This is useful to ensure travel stability of the vehicle at a high speed and to enhance maneuverability of the vehicle at a low speed.
In the four-wheel steering system, however, the coupling does not act to restrain movement of the rear-wheel steering mechanism either in a direction when the steering wheel is operated to rotate the front connecting shaft at a small angle. When the steering wheel is operated to rotate the front connecting shaft at a large angle, the coupling acts to restrain movement of the rear-wheel steering mechanism in one direction but does not act to restrain movement of the same in the other direction. For this reason, the rear-wheel steering mechanism is provided therein with resilient means for biasing the rear road wheels toward their neutral positions and for retaining the rear road wheels in their steered positions. In the rear-wheel steering mechanism, it is required to make the biasing force of the resilient means sufficient for resisting against a reaction force acting on the rear road wheels from road surfaces. As a result, the rear-wheel steering mechanism is complicated in construction and large in size. Moreover, the biasing force of the resilient means acts as a reaction force on the front connecting shaft when the steering wheel has been operated to rotate the front connecting shaft at a large angle. In this instance, the driver is obliged to increase his effort applied to the steering wheel and feels unstable in his steering operation.